Empty picture

[Pet]

No: HPFGUIDX 95040

Re-reading TOOTP last night I stumbled across something I'd completely
overlooked when I first read it. In the room that Ron and Harry are sharing
at Grimmauld Place there is an empty picture that's always sniggering
(Bloomsbury, p. 62, p. 151). Once the occupant even talks to Harry, telling
him that it's the "first sign of madness, talking to your own head" in a sly
voice (p. 162). 

So, we've been wondering if and in what form Sirius will show up again. What
if the invisible occupant is teenage Sirius? Lurking just out of sight? 
JKR wouldn't bother to mention the empty picture at least 4 times (once the
occupant is seemingly asleep, p.92) if it wasn't important, would she? 
Any thoughts?

Pet